1. Field of the Invention
The present invention relates generally to the field of medical devices for implantation into humans. More particularly, the present invention concerns methods for processing biological materials for use as bioprosthetic implantable devices.
2. Description of the Related Art
Bioprostheses are devices derived from processed biological tissues to be used for implantation into humans. The development of such devices originated as an attempt to circumvent some of the clinical complications associated with the early development of the mechanical heart valve, and has since resulted in a rapid proliferation of bioprosthetic devices for a variety of applications. Examples of some of the bioprostheses currently used or under development include heart valves, vascular grafts, biohybrid vascular grafts, ligament substitutes, pericardial patches, and others.
The primary component of the biological tissues used to fabricate bioprostheses is collagen, a generic term for a family of related extracellular proteins. Collagen molecules consist of three chains of poly(amino acids) arranged in a trihelical configuration ending in non-helical carboxyl and amino termini. These collagen molecules assemble to form microfibrils, which in turn assemble into fibrils, resulting in collagen fibers. The amino acids which make up the collagen molecules contain side groups, including amine (NH2), carboxylic acid (COOH) and hydroxyl (OH) groups, in addition to the amide bonds of the polymer backbone, all of which represent sites for potential chemical reaction on these molecules.
Because collagenous tissues degrade rapidly upon implantation into a host recipient, it is necessary to stabilize the tissue if it is to be used for clinical applications. Chemical stabilization by tissue cross-linking, also known as tissue fixation, has been achieved using a variety of compounds. Most typically, chemical fixation has employed polyfunctional molecules having two or more reactive groups capable of forming irreversible and stable intramolecular and intermolecular chemical bonds with the reactive amino acid side groups present on the collagen molecules. The most widely used of these polyfunctional molecules is the five carbon molecule, glutaraldehyde, which has an aldehyde at each end of a linear aliphatic chain. The aldehyde groups of glutaraldehyde and other like molecules react under physiological conditions with the primary amine groups of collagen molecules to cross-link the material. Glutaraldehyde cross-linked tissue produced in this way exhibits increased resistance to enzymatic degradation, reduced immunogenicity, and increased stability.
Despite its widespread use, there are certain disadvantages associated with tissue cross-linking with polyfunctional aldehydes and other chemical cross-linking agents. For example, upon implantation, aldehyde fixed tissue is susceptible to the formation of degenerative calcific deposits. Pathologic calcification, e.g., the undesirable deposition of calcium phosphate mineral salts in an implanted tissue, may represent the predominant cause of failure of glutaraldehyde-fixed bioprosthetic devices (Golomb et al., 1987; Levy et al., 1986; Thubrikar et al., 1983; Girardot et al., 1995). The mechanism for pathological calcification of implanted tissue is not fully understood, but may be due to host factors, implant factors, and/or extraneous factors, such as mechanical stress. Additionally, there is some evidence to suggest that deposits of calcium may be related to devitalized cells, and, in particular, to cell membranes in which the calcium pumps (Ca+2-Mg+2 ATPase) responsible for maintaining low intracellular calcium levels are no longer functioning or are malfunctioning.
Detergent pretreatment with non-covalently linked detergents, such as sodium dodecyl sulfate (SDS), or covalently bound detergents, such as amino oleic acid, have been reported to reduce calcification of materials exposed to circulating blood (Gott et al., 1992). However, detergents can adversely affect tissue structure and/or properties, resulting in a diminution of the collagen denaturation temperature, or shrink temperature, which is an important measure of material strength, durability, and integrity. Moreover, use of detergents can result in local toxicity.
In another approach, U.S. Pat. No. 5,746,775 describes the treatment of glutaraldehyde pretreated tissue with lower alcohols (i.e., C1-C3 alcohols), in which the lower alcohol is present at greater than 50% by volume in an alcohol treatment solution. The method is reported to be useful in preparing tissue for implantation into a living being.
Despite previous attempts at providing biomaterials having resistance to calcification, there remains a need for. alternative anticalcification approaches with improved efficacy and ease of use. There is, thus, a need for an effective method of imparting long-term anticalcification properties to bioprosthetic materials, e.g., tissues, that is not accompanied by deleterious effects and that incorporate anticalcification agents and/or treatments into existing protocols for the preparation of clinical-grade biomaterials. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.
According to one aspect of the present invention, there is provided a method for treating a biomaterial comprising contacting a biomaterial, such as a cross-linked animal tissue, with an anticalcification treatment solution. The anticalcification treatment solutions of this aspect of the invention include solutions comprised higher alcohols or polyols and polar aprotic organic solvents. The anticalcification treatment solutions are contacted with the biomaterial under conditions effective to reduce pathologic calcification of the biomaterial following implantation into a mammalian host. As illustrated herein, this reduction in calcification can be monitored, for example, by evaluating the calcium content of an implanted biomaterial treated with an anticalcification treatment solution of the invention compared with an implanted biomaterial not so treated. Preferably, this reduction in calcification will be greater than 50%, more preferably greater than 75%, and most preferably greater than 90%, compared with an implanted, untreated biomaterial.
The higher alcohol or polyol used in formulating the anticalcification treatment solution may be a linear or branched C4-C36 alcohol or polyol. In certain preferred embodiments of the invention, the higher alcohol or polyol will be selected from a C6-C18 alcohol or polyol, preferably from a C7-C9 alcohol or polyol. Typically, the higher alcohol or polyol comprises less than about 50% by volume of said anticalcification treatment solution. In some instances, however, it will be desired to use an anticalcification treatment solution wherein the higher alcohol or polyol comprises less than about 25% by volume of said anticalcification treatment solution, or even less than about 10% by volume of said anticalcification treatment solution. The anticalcification treatment solution of the present invention may further comprise at least one organic solvent selected from, for example, C1-C3 alcohols. Moreover, the anticalcification treatment solution can also comprise water or an aqueous solvent.
Polar aprotic organic solvents useful in formulating the anticalcification treatment solutions of the present invention will preferably have dielectric constants greater than about 20, more preferably greater than about 30, and they will possess some degree of water solubility. Polar aprotic organic solvents useful in this aspect of the invention include, for example, N-alkyl pyrolidinones and N-alkyl amides, in which the alkyl group or groups comprise branched or linear alkyl chains having from about 1 to 10 carbon atoms. Illustrative solvents of this class include N-methyl pyrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, N,N-dimethylpropionamide, and the like.
In a further aspect of the present invention, there is provided a method for treating an aldehyde cross-linked animal tissue by forming an anticalcification treatment solution comprised of at least one organic solvent and from about 0.1% to about 25% by volume of a C6-C18 alcohol or polyol, and contacting the anticalcification treatment solution with the aldehyde cross-linked biomaterial under conditions effective to reduce pathologic calcification of the biomaterial following implantation into a mammalian host. As described above, an anticalcification treatment solution of the invention may contain one or more organic solvents and may further comprise water or a compatible aqueous solvent system. In one illustrative embodiment of this aspect of the invention, an organic solvent is present at about 35% to about 49% by volume of said anticalcification treatment solution, the remainder being comprised of said water or aqueous solvent. In this embodiment, it is preferred that the water or aqueous solvent is present at greater than about 50% by volume of said anticalcification treatment solution.
In yet a further aspect of the present invention, there is provided a method for treating an aldehyde cross-linked mammalian tissue by providing an anticalcification treatment solution comprised of about 0.1% to about 25% by volume of a C6-C18 alcohol or polyol, about 25% to about 99% by volume of an organic solvent selected from a C1-C3 alcohol, the remaining volume, if any, being comprised of water or an aqueous solvent; and contacting the anticalcification treatment solution with an aldehyde cross-linked biomaterial for a duration effective to reduce pathologic calcification of the biomaterial following implantation into a mammalian host. One illustrative embodiment of this aspect of the invention employs an organic solvent that is present at about 35% to about 45% by volume of the anticalcification treatment solution and a higher alcohol or polyol that is present at about 1% to about 10% by volume of the anticalcification treatment solution.
In another aspect of this invention, a method is provided for treating a biomaterial, comprising contacting an aldehyde-cross-linked biomaterial with an anticalcification treatment solution comprised of N-methyl pyrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide and/or N,N-dimethylpropionamide under conditions effective to reduce pathologic calcification of the biomaterial following implantation into a mammalian host.
In another aspect of this invention, a method is provided for treating a biomaterial, preferably a cross-linked animal tissue, by contacting a biomaterial with an anticalcification treatment solution at a temperature between about 30xc2x0 and 60xc2x0 C. for a duration and under conditions effective to reduce pathologic calcification of the biomaterial following implantation into a mammalian host. The anticalcification treatment solutions comprise between about 10% and about 50% by volume, preferably between about 25% and 50% by volume, of a C1-C3 alcohol, such as methanol, ethanol, propanol, or isopropanol, the remaining volume being comprised of water or an aqueous buffer, such as HEPES.
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers"" specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The term xe2x80x9cbiomaterialxe2x80x9d is used herein to refer generally to collagen-containing, biologically-derived materials. For example, various types of implantable biological tissues derived from numerous animal sources and parts of the anatomy can be used as biomaterials in accordance with this invention. The tissue can be derived, for example, from animal sources such as human, bovine, porcine, equine, sheep, kangaroo, rabbit, and others. Illustrative examples of animal tissues used in accordance with the present invention include, without limitation, heart valves, particularly porcine or bovine heart valves; aortic roots, walls, and/or leaflets; pericardium; connective tissue-derived materials such as dura mater; homograft tissues such as aortic homografts and saphenous bypass grafts; tendons; ligaments; skin patches; arteries; veins; and the like, Of course, other biologically-derived materials that are known as being suitable for in-dwelling uses in the body of a living being are also within the contemplation of the invention. For some applications, it may be desired to manipulate the biomaterial in some manner so as to provide it in a particular form or shape, for example using metallic stents prior to the treatments described herein. In this way, the biomaterial may be cross-linked and/or alcohol treated in the particular three-dimensional geometric configuration of the bioprosthesis to be implanted.
Typically, the biomaterial treated according to this invention is comprised of a biomaterial that has been fixed/cross-linked by treatment with one or more chemical cross-linking agents or other treatments that effect cross-linking. These can include, for example, treatments with polyfunctional aldehydes, polyfunctional epoxides, photoxidation and/or any other cross-linking agents or treatments that promote reactions between carboxylic acid and amine groups, such as N-(3-Dimethylaminopropyl)-Nxe2x80x2-ethylcarbodiimide hydrochloride (EDC). Of course, the anticalcification treatments of this invention are preferably used in conjunction with cross-linking agents or treatments that increase the propensity of a biomaterial to calcify following implantation into a living host.
In one embodiment of the invention, the biomaterial is one that has been cross-linked by treatment with a monofunctional aldehyde, a polyfunctional aldehyde, or some combination thereof. A xe2x80x9cmonofunctional aldehydexe2x80x9d refers to a molecule containing a single aldehyde functionality, such as formaldehyde, while xe2x80x9cpolyfunctional aldehydexe2x80x9d refers to a molecule that contains two or more aldehyde functionalities. The other constituents present on the monofunctional or polyfunctional aldehyde are not critical provided they do not adversely effect the ability of the aldehyde groups to be collagen-reactive, and thereby capable of producing cross-linked biological tissues. Examples of monofunctional and polyfunctional aldehydes commonly used in biomaterial fixation methods for producing cross-linked biomaterials include aldehyde compounds that contain an aliphatic component comprising a linear or branched, saturated or unsaturated aliphatic chain having from about 2 to about 36 carbon atoms. Most preferably, cross-linking processes employ the use of a polyfunctional aldehyde having from 2 to about 10 carbon atoms, such as the linear five-carbon alkyl dialdehyde, glutaraldehyde.
As used herein, the terms xe2x80x9caldehyde fixed biomaterialxe2x80x9d or xe2x80x9caldehyde cross-linked biomaterialxe2x80x9d refers to biomaterial that has been treated with one or more monofunctional and/or polyfunctional aldehyde compounds. The techniques and coriditions for treating biomaterials with aldehyde-containing cross-linking agents are well known and are readily available to the skilled individual in the art (for example, see Zilla et al.). In these processes, a biomaterial is typically contacted with an aldehyde solution for a duration and under conditions effective to result in the desired degree of cross-linking of collagen and other cellular proteins within the tissue. Procedures for monitoring the progress and/or completion of the cross-linking reaction are also well known. For example, the degree of cross-linking of a treated tissue can be monitored by evaluating its shrinkage temperature and/or the quantity of extractable protein present in the material.
The skilled individual in this art will recognize that the duration of the cross-linking reaction according to this invention is not critical so long as the biomaterial and the cross-linking agent remain in contact for a time sufficient to allow cross-linking to occur. Time of treatment will of course vary depending on the type of biomaterial being treated, the particular aldehyde used and/or the concentration of the aldehyde in the cross-linking solution. Typically, the length of the reaction will be from about one minute to several days. However, the time of treatment should not be so long as to adversely effect the cross-linked biomaterial. Cross-linking times of several days or more are not uncommon. However, the biomaterial can also be treated for shorter periods as well, e.g., from about one minute to about twelve hours, or for about one hour to about six hours, provided the desired degree of cross-linking is achieved.
The reaction temperatures and/or pressures employed in a typical cross-linking reaction are not critical and can include essentially any conditions that are effective for allowing the cross-linking reaction to occur while not adversely compromising the progression of the reaction or the integrity of the biomaterial being treated. Identification of optimal temperature and pressure conditions for a particular implementation of the present invention can be readily determined by the skilled individual in this art. Generally, the cross-linking reaction can be carried out at an ambient temperature, or at any other convenient temperature that does not substantially exceed the tissue denaturation temperature of about 62xc2x0 C. Thus, reaction temperatures may be selected from a temperature range from about 0xc2x0 C. to about 60xc2x0 C., preferably from about 20xc2x0 C. to about 50xc2x0 C. Although the pressure for a typical reaction generally ranges from about 2 mm Hg to about 6 mm Hg, suitable pressures may be as high as 100 mm Hg, or more, if desired.
After the biomaterial is cross-linked in this manner, the tissue is optionally washed/rinsed, and is contacted with an anticalcification treatment solution. The anticalcification treatment solutions of the present invention include solutions comprised of higher alcohols, polyols (i.e., organic molecules containing two more alcohol functionalities), polar aprotic solvents, such as N-methyl pyrolidinone, and solutions comprised of less than about 50% by volume of one or more lower (C1-C3) alcohols.
Therefore, according to one embodiment of the present invention, the anticalcification treatment solutions is comprised of one or more higher alcohols or polyols (e.g., a C4 to C36 alcohol or polyol). The higher alcohol or polyol will typically be an aliphatic linear or branched alcohol or polyol, and may contain additional chemical moieties or substituents provided they do not unacceptably interfere with the anticalcification effects described herein. In one illustrative embodiment of the invention, the higher alcohols used to formulate and anticalcification treatment solution are primary, secondary or tertiary alcohols selected from linear or branched C6-C18 aliphatic alcohols, such as hexanol, heptanol, octanol, nonanol, etc., or linear or branched C6-C18 polyols selected from 1,2-octanediol (also sometimes referred to as 1,2-dihydroxyoctane), 1,8-octanediol, 1,10-decanol, 1,10-dodecanol, 1,2-dihydroxydecane and 1,2-dihydroxydodecane.
In certain illustrative embodiments of the invention, the higher alcohols or polyols are present at less than about 50%, less than about 25%, or less than about 10%, by volume of the anticalcification treatment solution, the remainder being comprised of an organic solvent. Thus, in addition to the higher alcohols and polyols described above, the anticalcification treatment solution of the present invention may further contain one or more organic solvents. The organic solvents used in accordance with the present invention are preferably selected from those that do not have deleterious effects on the tissue being treated or on the anticalcification effects achieved by use of the anticalcification treatment solution. The organic solvents should be capable of adequately dissolving the higher alcohol or polyol to form a homogeneous anticalcification treatment solution. Organic solvents that can improve, enhance, or otherwise facilitate the anticalcification effects of the higher alcohols or polyols of this invention are, of course, particularly preferred. Organic solvents useful in accordance with this embodiment include lower alcohols (e.g., C1-C3 alcohols), acetone, ethyl acetate, ethyl lactate, 1,4-butanediol, polyethylene glycol, and the like.
Anticalcification treatment solutions according to certain embodiments of the invention comprise one or more higher alcohols and/or polyols in a preferably homogeneous mixture with one or more organic solvents. For example, particularly illustrative alcohol treatment solutions comprise from about 0.1% to about 25% by volume of one or more higher alcohols or polyols, with substantially all of the remainder of said solution being comprised of organic solvent. Additional illustrative anticalcification treatment solutions comprise from about 0.1% to about 10% by volume of one or more higher alcohols or polyols, with substantially all of the remainder of the solution being comprised of organic solvent.
Alternatively, the one or more higher alcohols or polyols of the anticalcification treatment solution may be formulated in an aqueous solvent system, e.g., with water or with any of a variety of aqueous buffer systems, or may be formulated in a mixture of an aqueous solvent system and one or more organic solvents. Some higher alcohols and polyols may exhibit poor solubility in aqueous based systems, but have greater solubility in many organic solvents. Thus, in embodiments which employ an aqueous based solvent systems, it will in some instances be preferred that one or more organic solvents is also employed in an amount at least sufficient to dissolve the higher alcohol or polyol to provide a homogeneous, i.e., substantially single-phase, anticalcification treatment solution.
Therefore, in additional embodiments of the invention, anticalcification treatment solutions are comprised of about 0.1% to about 25% by volume of one or more higher alcohols or polyols, about 25% to about 49% by volume of one or more organic solvents, with substantially all of the remainder of said solution being water or an aqueous based solution. Further embodiments of the invention provide anticalcification treatment solution comprised of about 0.1% to about 10% by volume of one or more higher alcohols or polyols, about 35% to about 45% by volume of one or more organic solvents, with substantially all of the remainder of said solution being water or an aqueous based solution.
In another embodiment of this invention, the anticalcification treatment solution is comprised of one or more polar aprotic solvents. Such solvents can include, for example, N-alkyl pyrolidinones and N-alkyl amides, in which the alkyl group or groups comprise linear or branched alkyl chains having from about 1 to 10 carbon atoms. Illustrative solvents of this type include N-methyl pyrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, N,N-dimethylpropionamide, and the like.
Particularly preferred polar aprotic solvents include those having some degree of water solubility and/or those with high dielectric constants, for example having dielectric constants greater than about 20, preferably greater than about 30.
In yet another embodiment of the invention, lower (C1-C3) alcohol treatment solutions, comprising less than 50% by volume of the lower alcohol, preferably between about 25% and 50%, are also suitable as anticalcification treatment solutions. Whereas prior anticalcification treatment attempts using lower alcohol solutions such as these have been unsuccessful, it has now been found that significant anticalcification effects can indeed be achieved by contacting a biomaterial with a lower alcohol treatment solution at a temperature in the range of about 30xc2x0 C. to about 60xc2x0 C., preferably between about 35xc2x0 C. and 45xc2x0 C. These treatment temperatures improve the efficacy of the anticalcification treatment solutions of this embodiment, possibly by facilitating the diffusion and penetration of the lower alcohols into the biomaterial. Preferably, the treatment according to this embodiment is accompanied by agitation of the anticalcification treatment solution while it is in contact with the biomaterial.
Cross-linked biomaterial is contacted with, or otherwise exposed to, an anticalcification treatment solution of the present invention for a period of time sufficient to render the biomaterial more resistant to in vivo pathologic calcification than a biomaterial not treated with the anticalcification treatment solution. The length of exposure in the embodiments described herein is illustrative only and can be varied by those of skill in the art while achieving a desired result. For embodiments of the invention wherein the biomaterial is immersed or soaked in a liquid anticalcification treatment solution, the exposure time will typically be in the range of about 1 hour to about 96 hours. For some biomaterials, excessive exposure to the anticalcification treatment solution may result in a decrease in the anticalcification effects, or may necessitate rehydration of the tissue.
The treatment procedure can be carried out at or near room temperature (e.g., about 25xc2x0 C.) if desired. However, any temperature of convenience that is not deleterious to the biomaterial, for example about 4xc2x0 C. to about 60xc2x0 C., may also be used. As discussed above, it may indeed be desired and/or necessary in some embodiments to use an incubation temperature greater than room temperature in order to improve the efficacy of the treatment process, for example by increasing the rate and/or degree of diffusion and penetration of the anticalcification solutions into the biomaterial.
The biomaterial will typically be treated by contact with a liquid anticalcification treatment solution. However, other approaches could also be taken, such as vapor, plasma, and/or cryogenic application. Irrespective of the method of exposure, the time period should be sufficient to inhibit calcification, but not so long as to cause irreparable dehydration of the tissue by any of the constituents of the anticalcification treatment solution. In certain embodiments, the biomaterial is shaken or otherwise agitated during exposure to the anticalcification treatment solution in order to facilitate greater penetration of the constituents of the solution into the biomaterial. Shaking can be accomplished in any convenient manner, such as through use of an orbital shaker or shaker stand, or by manual agitation.
In some instances, it will be preferred to formulate an anticalcification treatment solution that is buffered in an aqueous solvent system, for example to a pH between about 6.0 and 8.0, preferably to a pH between about 7.0 and 7.6. Suitable buffers for use in this regard include buffers which have a buffering capacity sufficient to maintain a physiologically acceptable pH and do not cause any deleterious effects to the biomaterial or interfere with the treatment process being performed. Illustrative buffers include phosphate-buffered saline (PBS), organic buffers, such as N-N-2-hydroxyethylpiperzine-Nxe2x80x2-2-ethanesulfonic acid (HEPES) and morpholine propanesulphonic acid (MOPS), and buffers which include borate, bicarbonate, carbonate, cacodylate, and the like. Many additional aqueous and other buffering systems suitable for use in the present invention will be apparent to the skilled artisan.
The biomaterial that has been treated with an anticalcification treatment solution may be rinsed prior to implantation or storage to remove any undesired and/or deleterious components produced or used in the biomaterial treatment protocol, such as cellular debris or aldehyde fragments from an aldehyde pretreatment. As used herein, the term xe2x80x9crinsexe2x80x9d includes subjecting the biomaterial to a rinsing solution, including continuously or by batch processing, wherein the biomaterial is placed in a rinsing solution which may be periodically removed and replaced with fresh solution at predetermined intervals. During rinsing, the tissue is preferably shaken, or intermittently stirred, to ensure even distribution of the rinse solution. Illustratively, a rinse may comprise soaking the biomaterial in fresh rinsing solution which is replaced several times over a period of about an hour or less. Alternatively, the rinsing solution may be replaced at intervals of several hours or more over a longer rinse period, such as about 24 hours. Exemplary rinsing solutions include physiologically suitable solutions, such as water, saline, PBS, HEPES buffered saline, ringers lactate (pH 7.4), sodium bicarbonate (pH 7.4), tris (pH 7.4), imidazole (pH 7.4), and the like.
Subsequent to rinsing, the treated biomaterial is ready for implantation or may be sterilized and stored until use. Storage in standard glutaraldehyde solutions of the type typically used for long-term storage of clinical-grade bioprostheses may partially reverse the beneficial effects achieved by the treatment method of the present invention. Thus, it may be advantageous to store the treated biomaterial in an alcohol- or polyol-containing solution, such as an alcohol-glutaraldehyde solution, preferably under conditions which maintain calcification inhibition properties of the treated material.
In other embodiments of the invention, biomaterials which have been treated in accordance with the method of the invention are stored in an aldehyde-free environment. For example, treated tissue may be placed in sterile bags and subjected to sterilizing radiation, such as gamma-radiation. Of course, the treatment method of the present invention will be compatible with many other known sterilizing preservatives and/or techniques which are known by those of skill in the art.
In additional embodiments, the anticalcification treatment solution of the present invention may further comprise one or more additional anticalcification agents, including but not limited to, a soluble salt of a metallic cation, such as Al+3 or Fe+3, preferably in a concentration range of 0.001M to 0.1M. Water soluble aluminum salts, for example, which are suitable additional anticalcification agents for use in the practice of the present invention, include without limitation, aluminum chlorate, aluminum lactate, aluminum potassium sulfate, aluminum sodium sulfate, aluminum sulfate, aluminum nitrate, and aluminum chloride. Also, water-soluble ferric salts, such as ferric chloride, ferric nitrate, ferric bromide, ferric sodium edentate, ferric sulfate, and ferric formate, are also within the contemplation of the invention. Of course, any salt of aluminum, or iron, which is soluble in the solvent system of the treatment solution, may be used in the practice of the invention.
Although not wishing to be bound by this theory, the following may explain, at least in part, certain advantages realized by employing anticalcification treatment solutions in accordance with the present invention. In living tissue and cells, the typical extracellular calcium concentration is about 1 mM and the intracellular calcium concentration is about 0.1 xe2x96xa1M. This large concentration gradient of calcium between the extracellular and intracellular regions is maintained by biochemical metabolic energy-dependent pumps across the plasma membranes of cells. Upon fixation, these biochemical forces are not active, and this results in a high concentration of calcium throughout the fixed tissue matrix. Plasma membranes and membrane bound organelles are rich in phospholipids, which provide phosphorous for calcium phosphate formation. In the in vivo environment, the high concentration of calcium in the fixed tissue coupled with a source of phosphorous from lipids may favor conditions for calcium phosphate crystallization. However, the constituents of the anticalcification treatment solutions used in accordance with this invention can be highly effective in penetrating the tissue matrix, interacting with, and possibly facilitating the removal of, phospholipids and other cellular debris from the cross-linked biomaterial, thereby interfering with the ability of such components to contribute to the crystallization process.